Fusion formable silica and sodium containing glasses

ABSTRACT

Sodium containing aluminosilicate and boroaluminosilicate glasses are described herein. The glasses can be used as substrates or superstrates for photovoltaic devices, for example, thin film photovoltaic devices such as CIGS photovoltaic devices. These glasses can be characterized as having strain points ≧535° C., for example, ≧570° C., thermal expansion coefficients of from 8 to 9 ppm/° C., as well as liquidus viscosities in excess of 50,000 poise. As such they are ideally suited for being formed into sheet by the fusion process.

This application claims the benefit of priority to U.S. ProvisionalPatent Application No. 61/228,290 filed on Jul. 24, 2009, to U.S.Provisional Patent Application No. 61/263,930 filed on Nov. 24, 2009,and to U.S. Provisional Patent Application No. 61/347,589 filed on May24, 2010.

BACKGROUND

1. Field

Embodiments relate generally to sodium containing glasses and moreparticularly to fusion formable silica and sodium containing glasseswhich may be useful in photochromic, electrochromic, Organic LightEmitting Diode (OLED) lighting, or photovoltaic applications, forexample, thin film photovoltaics.

2. Technical Background

Recent interest in the higher efficiencies offered by thin filmphotovoltaics has spawned considerable efforts into the development ofnew glass substrates and superstrates tailored to the needs of this newmarket. The thin film photovoltaics manufacturing processes typicallyrequire substrates capable of handling elevated temperatures forextended periods of time without warping, making glasses particularlywell suited for these applications. Additionally, some thin filmphotovoltaic processes (such as CIGS) desire sodium to diffuse from theglass into the deposited layers, making sodium-containing glasses evenmore desirable for particular applications.

Existing glasses (such as soda lime or display compositions) have beenused to demonstrate extremely high efficiencies in this field but theuse of any glasses designed for other applications is wrought withproblems. For example, soda lime glass offers a cheap, readily availablesodium-containing substrate but its low strain point drasticallyinhibits its use in the higher temperature process that allow thin filmphotovoltaic processes to reach their highest efficiencies.

The use of glasses designed for display applications provides therequired high strain point but the coefficient of thermal expansion(CTE) of these glasses is often too low to allow the reliableconstruction of large photovoltaic panels due to CTE mismatch with thephotovoltaic films. Additionally, many glasses designed for displayapplications are intentionally alkali-free, making them less useful forthose thin film photovoltaic applications desiring sodium diffusion fromthe glass.

In some thin film photovoltaic applications, it would be advantageous tohave a sodium-containing glass sheet with a high strain point and a highCTE. Further, it would be advantageous to have a sodium-containing glasswith a high strain point and high CTE that is fusion formable to allowprocessing into a flat sheet with optimal surface characteristics.

SUMMARY

A compositional range of fusion-formable, high strain pointsodium-containing aluminosilicate and boroaluminosilicate glassesuseful, for example, for thin-film photovoltaic applications aredescribed herein. More specifically, these glasses are advantageousmaterials to be used in copper indium gallium diselenide (CIGS)photovoltaic modules where the sodium required to optimize cellefficiency is to be derived from the substrate glass. Current CIGSmodule substrates are typically made from soda-lime glass sheet that hasbeen manufactured by the float process. However, use of higher strainpoint glass substrates can enable higher temperature CIGS processing,which is expected to translate into desirable improvements in cellefficiency. Moreover, it may be that the smoother surface offusion-formed glass sheets yields additional benefits, such as improvedfilm adhesion, etc.

Accordingly, the sodium-containing glasses described herein can becharacterized by strain points ≧540° C., for example, ≧570° C. so as toprovide advantage with respect to soda-lime glass and/or liquidusviscosity ≧50,000 poise to allow manufacture via the fusion process, forexample, a liquidus viscosity of 130,000 poise or greater. In order toavoid thermal expansion mismatch between the substrate and CIGS layer,the inventive glasses, according to some embodiments, are furthercharacterized by a thermal expansion coefficient in the range of from 8to 9 ppm/° C.

One embodiment is a glass comprising, in weight percent:

-   -   50 to 72 percent SiO₂;    -   greater than 15 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   greater than 0 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

Another embodiment is a glass comprising, in weight percent:

-   -   50 to 72 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   0.5 to less than 14 percent RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

Another embodiment is a photovoltaic device comprising, a glasscomprising, in weight percent:

-   -   50 to 72 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   greater than 0 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from the description or recognizedby practicing the invention as described in the written description andclaims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework tounderstanding the nature and character of the invention as it isclaimed.

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate one or moreembodiment(s) of the invention and together with the description serveto explain the principles and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be understood from the following detailed descriptioneither alone or together with the accompanying drawings.

FIG. 1 is an illustration of features of a photovoltaic device accordingto one embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of theinvention.

As used herein, the term “substrate” can be used to describe either asubstrate or a superstrate depending on the configuration of thephotovoltaic cell. For example, the substrate is a superstrate, if whenassembled into a photovoltaic cell, it is on the light incident side ofa photovoltaic cell. The superstrate can provide protection for thephotovoltaic materials from impact and environmental degradation whileallowing transmission of the appropriate wavelengths of the solarspectrum. Further, multiple photovoltaic cells can be arranged into aphotovoltaic module. Photovoltaic device can describe either a cell, amodule, or both.

As used herein, the term “adjacent” can be defined as being in closeproximity. Adjacent structures may or may not be in physical contactwith each other. Adjacent structures can have other layers and/orstructures disposed between them.

One embodiment is a glass comprising, in weight percent:

-   -   50 to 72 percent SiO₂;    -   greater than 15 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   greater than 0 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

Another embodiment is a glass comprising, in weight percent:

-   -   50 to 72 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   0.5 to less than 14 percent RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

Another embodiment is a photovoltaic device comprising, a glasscomprising, in weight percent:

-   -   50 to 72 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   greater than 0 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

In another embodiment, the photovoltaic device comprises a glassconsisting essentially of, in weight percent:

-   -   50 to 72 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   greater than 0 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

The photovoltaic device can comprise any of the described embodiments ofthe disclosed glasses. The glass can be in the form of a sheet and beeither the substrate or superstrate or both of the photovoltaic device.

In another embodiment, the glass comprises, in weight percent:

-   -   50 to 72 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   greater than 0 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, Li, Rb, and Cs whereinthe glass comprises substantially no K₂O and wherein the glass comprises9 to 17 weight percent Na₂O, and wherein, R is an alkaline earth metalselected from Mg, Ca, Ba, and Sr.

In another embodiment, the glass consists essentially of, in weightpercent:

-   -   50 to 72 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   greater than 0 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, Li, Rb, and Cs whereinthe glass comprises substantially no K₂O and wherein the glass comprises9 to 17 weight percent Na₂O, and wherein, R is an alkaline earth metalselected from Mg, Ca, Ba, and Sr.

In one embodiment, the glass comprises, in weight percent:

-   -   50 to 59 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   2 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

In another embodiment, the glass comprises, in weight percent:

-   -   54 to 59 percent SiO₂;    -   10 to 21 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   2 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

According to another embodiment, the glass comprises, in weight percent:

-   -   54 to 59 percent SiO₂;    -   17 to 21 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   2 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

According to another embodiment, the glass comprises, in weight percent:

-   -   52 to 59 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   2 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

In another embodiment, the glass consists essentially of, in weightpercent:

-   -   54 to 59 percent SiO₂;    -   17 to 21 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   2 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O; and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

In a further embodiment, the glass comprises, in weight percent:

-   -   56 to 58 percent SiO₂;    -   17 to 21 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   2 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

In another embodiment, the glass consists essentially of, in weightpercent:

-   -   50 to 59 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   2 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

In another embodiment, the glass consists essentially of, in weightpercent:

-   -   52 to 59 percent SiO₂;    -   10 to 25 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   2 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

In yet another embodiment, the glass consists essentially of, in weightpercent:

-   -   56 to 58 percent SiO₂;    -   17 to 21 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   2 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

In one embodiment, the glass consists essentially of, in weight percent:

-   -   54 to 59 percent SiO₂;    -   to 21 percent Al₂O₃;    -   0 to 10 percent B₂O₃;    -   10 to 25 percent total M₂O; and    -   2 to 25 percent total RO;

wherein, M is an alkali metal selected from Na, K, Li, Rb, and Cs andwherein the glass comprises at least 9 weight percent Na₂O, and wherein,R is an alkaline earth metal selected from Mg, Ca, Ba, and Sr.

According to one embodiment, the glass comprises 55 to 72 weight percentSiO₂, for example, 51 to 72 weight percent SiO₂, for example, 52 to 72weight percent SiO₂, for example, 53 to 72 weight percent SiO₂, forexample, 54 to 72 weight percent SiO₂, for example, 55 to 72 weightpercent SiO₂, for example, 56 to 72 weight percent SiO₂, for example, 57to 72 weight percent SiO₂, for example, 58 to 72 weight percent SiO₂,for example, 59 to 72 weight percent SiO₂, for example, 60 to 72 weightpercent SiO₂. In one embodiment, the glass comprises 55 to 72 weightpercent SiO₂ and comprises greater than 15 to 25 weight percent Al₂O₃.

The glass, in one embodiment, is rollable. The glass, in one embodiment,is down-drawable. The glass can be slot drawn or fusion drawn, forexample. According to another embodiment the glass can be float formed.

The glass, according to one embodiment, comprises less than 8 weightpercent K₂O, for example, less than 7 weight percent K₂O, for example,less than 6 weight percent K₂O, less than 5 weight percent K₂O, lessthan 4 weight percent K₂O, for example, less than 3 weight percent K₂O.The glass, according to some embodiments, comprises substantially noK₂O, for example, is substantially K₂O free.

According to one embodiment, the glass comprises less than 4 weightpercent K₂O, and the glass has a strain point of 535° C. or greater, acoefficient of thermal expansion of 50×10⁻⁷ or greater, and has aliquidus viscosity of 130,000 poise or greater, for example, 150,000poise or greater. The glass having these properties, in one embodiment,is fusion formable.

The glass, according to one embodiment, comprises less than 4 weightpercent K₂O and less than 2.5 weight percent MgO. In one embodiment, theglass comprises less than 4 weight percent K₂O and less than 2.5 weightpercent MgO and has a strain point of 535° C. or greater, a coefficientof thermal expansion of 50×10⁻⁷ or greater, and a liquidus viscosity of130,000 poise or greater, for example, 150,000 poise or greater. Theglass having these properties, in one embodiment, is fusion formable.

Some embodiments of the disclosed glasses have the advantage of a highNa₂O content, making them capable of delivering more Na to a depositedCIGS layer during fabrication of photovoltaic cells—which in turn isexpected to lead to higher CIGS cell efficiency. Finally, as Naoutdiffusion during CIGS deposition/crystallization may possibly beimpeded by the presence of another alkali, the fact that some examplesare K-free or have substantially reduced K₂O content may provide yetanother advantage.

The glass can further comprise 3 weight percent or less, for example, 0to 3 weight percent, for example, greater than 0 to 3 weight percent,for example, 1 to 3 weight percent of TiO₂, MnO, ZnO, Nb₂O₅, MoO₃,Ta₂O₅, WO₃, ZrO₂, Y₂O₃, La₂O₃, HfO₂, CdO, SnO₂, Fe₂O₃, CeO₂, As₂O₃,Sb₂O₃, Cl, Br, or combinations thereof. The glass, in one embodiment,comprises 3 weight percent or less, for example, 0 to 3 weight percent,for example, greater than 0 to 3 weight percent, for example, 1 to 3weight percent of TiO₂ or ZrO₂.

As mentioned above, the glasses, according some embodiments, comprise 0to 10 weight percent, for example, 1 to 8 weight percent or for example,greater than 0 to 10 weight percent B₂O₃, for example, 0.5 to 10 weightpercent B₂O₃, for example 1 to 10 weight percent B₂O₃. B₂O₃ is added tothe glass to reduce melting temperature, to decrease liquidustemperature, to increase liquidus viscosity, and to improve mechanicaldurability relative to a glass containing no B₂O₃. In one embodiment,the glass is substantially B₂O₃ free.

The glass, according to one embodiment, comprises greater than 0 to 25percent RO, for example, 0.5 to 25 percent RO, for example, 1 to 25percent RO wherein, R is an alkaline earth metal. The glass, accordingto one embodiment, comprises less than 14 percent RO, for example, 13 orless, for example, 12 or less, for example, 11 or less, for example, 10or less, for example, 9 or less, for example, 8 or less. In oneembodiment, the glass comprises 0.5 to less than 14 percent RO, forexample, 0.5 to 13 percent RO. The glass, according to one embodiment,comprises greater than 2 to 25 percent RO, for example, wherein R is analkaline earth metal.

According to one embodiment, the glass comprises 0.5 to less than 14percent RO, and the glass has a strain point of 535° C. or greater, acoefficient of thermal expansion of 50×10⁻⁷ or greater, and has aliquidus viscosity of 130,000 poise or greater, for example, 150,000poise or greater. The glass having these properties, in one embodiment,is fusion formable.

The glass, according to some embodiments, comprises less than 4.0 weightpercent MgO, for example, less than 3.0 weight percent MgO, for example,less than 2.5 weight percent MgO, less than 2.0 weight percent MgO. Theglass can comprise, for example, 0 to 4 weight percent MgO, for example,greater than 0 to 4 weight percent MgO, for example, greater than 0 to 3weight percent MgO, for example, greater than 0 to 2.5 weight percentMgO, for example, 0.2 to 4 weight percent MgO, for example, 0.2 to 3weight percent MgO, for example, 0.2 to 2.5 weight percent MgO.According to another embodiment, the glass comprises, for example, 1 to3 weight percent MgO. MgO can be added to the glass to reduce meltingtemperature and to increase strain point. It can disadvantageously lowerCTE relative to other alkaline earths (e.g., CaO, SrO, BaO), and soother adjustments may be made to keep the CTE within the desired range.Examples of suitable adjustments include increase SrO at the expense ofCaO, increasing alkali oxide concentration, and replacing a smalleralkali oxide (e.g., Na₂O) in part with a larger alkali oxide (e.g.,K₂O).

According to one embodiment, the glass comprises less than 2.5 weightpercent MgO, and the glass has a strain point of 535° C. or greater, acoefficient of thermal expansion of 50×10⁻⁷ or greater, and has aliquidus viscosity of 130,000 poise or greater, for example, 150,000poise or greater. The glass having these properties, in one embodiment,is fusion formable.

According to another embodiment, the glass is substantially free of BaO.For example, the content of BaO can be 0.05 weight percent or less, forexample, zero weight percent.

In some embodiments, the glass is substantially free of Sb₂O₃, As₂O₃, orcombinations thereof, for example, the glass comprises 0.05 weightpercent or less of Sb₂O₃ or As₂O₃ or a combination thereof. For example,the glass can comprise zero weight percent of Sb₂O₃ or As₂O₃ or acombination thereof.

The glasses, in some embodiments, comprise 2 to 4 weight percent CaO.Relative to alkali oxides or SrO, CaO contributes to higher strainpoint, lower density, and lower melting temperature. It is a primarycomponent of certain possible denitrification phases, particularlyanorthite (CaAl₂Si₂O₈), and this phase has complete solid solution withan analogous sodium phase, albite (NaAlSi₃O₈). High Na and Ca contentstaken alone can cause liquidus temperatures to be unacceptably high.However, the chemical sources for CaO include limestone, a veryinexpensive material, so to the extent that high volume and low cost arefactors, it is typically useful to make the CaO content as high as canbe reasonably achieved relative to other alkaline earth oxides.

The glasses can comprise, in some embodiments, 0.2 to 4 weight percentSrO, for example, 0.5 to 4 weight percent, for example 1 to 4, forexample, 2 to 4 weight percent SrO. In certain embodiments, the glasscontains no deliberately batched SrO, though it may of course be presentas a contaminant in other batch materials. SrO contributes to highercoefficient of thermal expansion, and the relative proportion of SrO andCaO can be manipulated to improve liquidus temperature, and thusliquidus viscosity. SrO is not as effective as CaO or MgO for improvingstrain point, and replacing either of these with SrO tends to cause themelting temperature to increase.

Also as mentioned above, the glasses, according to some embodiments,include 10 to 25 percent M₂O, wherein M is one of the alkali cations Li,Na, K, Rb and Cs. The alkali cations raise the CTE steeply, but alsolower the strain point and, depending upon how they are added, increasemelting temperatures. The least effective alkali oxide for CTE is Li₂O,and the most effective alkali oxide is Cs₂O. As noted above, sodium canparticipate in one of the possible denitrification phases of theinventive glasses, and while adjustments in other components can be usedto counteract this, e.g., changing the CaO/(CaO+SrO) ratio, thistendency may make it advantageous to replace sodium with other alkalis,or to use a mix of alkalis instead of sodium alone. If high volume andlow cost are important, then it is desirable to as much as possibleconfine the alkali oxides to Na₂O and K₂O or combinations thereof.

According to some embodiments, the glass comprises 9 to 17 percent Na₂O,for example, 10 to 16 percent Na₂O. In one embodiment, the glasscomprises 9 weight percent or more Na₂O, for example, 9 to 12 weightpercent Na₂O.

The glass, according to some embodiments, is down-drawable; that is, theglass is capable of being formed into sheets using down-draw methodssuch as, but not limited to, fusion draw and slot draw methods that areknown to those skilled in the glass fabrication arts. Such down-drawprocesses are used in the large-scale manufacture of ion-exchangeableflat glass.

The glass, according to one embodiment, comprises 10 to 30 weightpercent Al₂O₃+B₂O₃.

The glass, according to one embodiment, comprises 20 to 30 weightpercent Al₂O₃+B₂O₃.

The glass, according to one embodiment, comprises 21 to 25 weightpercent Al₂O₃.

The glass, according to one embodiment, comprises 10 to 21 weightpercent Al₂O₃+B₂O₃.

The glass, according to one embodiment, comprises 17 to 21 weightpercent Al₂O₃+B₂O₃.

The glass, according to one embodiment, comprises greater than 15 to 25weight percent Al₂O₃, for example, 16 or greater to 25 weight percent,for example, 16 to 24 weight percent Al₂O₃ or, for example, 17 to 25weight percent Al₂O₃, for example, 17 to 21 weight percent Al₂O₃.

According to one embodiment, the glass comprises greater than 15 to 25weight percent Al₂O₃, has a strain point of 535° C. or greater, acoefficient of thermal expansion of 50×10⁻⁷ or greater, and a liquidusviscosity of 130,000 poise or greater, for example, 150,000 poise orgreater. The glass having these properties, in one embodiment, is fusionformable.

The glass, according to one embodiment, comprises greater than 15 to 25percent Al₂O₃ and comprises 0.5 to less than 14 percent RO. In oneembodiment, the glass comprises greater than 15 to 25 percent Al₂O₃, 0.5to less than 14 percent RO, has a strain point of 535° C. or greater, acoefficient of thermal expansion of 50×10⁻⁷ or greater, and a liquidusviscosity of 130,000 poise or greater, for example, 150,000 poise orgreater. The glass having these properties, in one embodiment, is fusionformable.

The glass, according to one embodiment, comprises:

-   -   9 to 12 percent Na₂O;    -   2 to 8 percent K₂O;    -   2 to 8 percent CaO;    -   2 to 4 percent SrO; and    -   1 to 3 percent MgO.

The fusion draw process uses a drawing tank that has a channel foraccepting molten glass raw material. The channel has weirs that are openat the top along the length of the channel on both sides of the channel.When the channel fills with molten material, the molten glass overflowsthe weirs. Due to gravity, the molten glass flows down the outsidesurfaces of the drawing tank. These outside surfaces extend down andinwardly so that they join at an edge below the drawing tank. The twoflowing glass surfaces join at this edge to fuse and form a singleflowing sheet. The fusion draw method offers the advantage that, sincethe two glass films flowing over the channel fuse together, neitheroutside surface of the resulting glass sheet comes in contact with anypart of the apparatus. Thus, the surface properties are not affected bysuch contact.

The slot draw method is distinct from the fusion draw method. Here themolten raw material glass is provided to a drawing tank. The bottom ofthe drawing tank has an open slot with a nozzle that extends the lengthof the slot. The molten glass flows through the slot/nozzle and is drawndownward as a continuous sheet therethrough and into an annealingregion. Compared to the fusion draw process, the slot draw processprovides a thinner sheet, as only a single sheet is drawn through theslot, rather than two sheets being fused together, as in the fusiondown-draw process.

In order to be compatible with down-draw processes, thealuminoborosilicate glass described herein has a high liquidusviscosity. In one embodiment, the glass has a liquidus viscosity of50,000 poise or greater, for example, 150,000 poise or greater, forexample, 200,000 poise or greater, for example, 250,000 poise orgreater, for example, 300,000 poise or greater, for example, 350,000poise or greater, for example, 400,000 poise or greater, for example,greater than or equal to 500,000 poise. The liquidus viscosities of someexemplary glasses could be closely correlated with the differencebetween the liquidus temperature and the softening point.

In one embodiment, the glass has a strain point of 535° C. or greater,for example, 540° C. or greater, for example, a strain point of 560° C.or greater, for example, a strain point of 570° C. or greater, forexample, 580° C. or greater. In some embodiments, the glass has acoefficient of thermal expansion of 50×10⁻⁷ or greater, for example,60×10⁻⁷ or greater, for example, 70×10⁻⁷ or greater, for example,80×10⁻⁷ or greater. In one embodiment, the glass has a coefficient ofthermal expansion of from 50×10⁻⁷ to 90×10⁻⁷.

In one embodiment, the glass has a strain point of 535° C. or greater, acoefficient of thermal expansion of 50×10⁻⁷ or greater, and has aliquidus viscosity of 150,000 poise or greater. The glass having theseproperties, in one embodiment, is fusion formable.

In one embodiment, the glass has a coefficient of thermal expansion of50×10⁻⁷ or greater and a strain point of 535° C. or greater. In oneembodiment, the glass has a coefficient of thermal expansion of 50×10⁻⁷or greater and a strain point of 540° C. or greater. In one embodiment,the glass has a coefficient of thermal expansion of 60×10⁻⁷ or greaterand a strain point of 560° C. or greater. In one embodiment, the glasshas a coefficient of thermal expansion of 60×10⁻⁷ or greater and astrain point of 580° C. or greater. In one embodiment, the glass has acoefficient of thermal expansion of 50×10⁻⁷ or greater and a strainpoint of 570° C. or greater. In one embodiment, the glass has acoefficient of thermal expansion of 70×10⁻⁷ or greater and a strainpoint of 570° C. or greater. Embodiments of the described glasses canhave several combinations of properties within the disclosed ranges. Itshould be appreciated that all of the possible combinations are notlisted herein.

According to one embodiment, the glass is ion exchanged in a salt bathcomprising one or more salts of alkali ions. The glass can be ionexchanged to change its mechanical properties. For example, smalleralkali ions, such as lithium or sodium, can be ion-exchanged in a moltensalt containing one or more larger alkali ions, such as sodium,potassium, rubidium or cesium. If performed at a temperature well belowthe strain point for sufficient time, a diffusion profile will form inwhich the larger alkali moves into the glass surface from the salt bath,and the smaller ion is moved from the interior of the glass into thesalt bath. When the sample is removed, the surface will go undercompression, producing enhanced toughness against damage. Such toughnessmay be desirable in instances where the glass will be exposed to adverseenvironmental conditions, such as photovoltaic grids exposed to hail. Alarge alkali already in the glass can also be exchanged for a smalleralkali in a salt bath. If this is performed at temperatures close to thestrain point, and if the glass is removed and its surface rapidlyreheated to high temperature and rapidly cooled, the surface of theglass will show considerable compressive stress introduced by thermaltempering. This will also provide protection against adverseenvironmental conditions. It will be clear to one skilled in the artthat any monovalent cation can be exchanged for alkalis already in theglass, including copper, silver, thallium, etc., and these also provideattributes of potential value to end uses, such as introducing color forlighting or a layer of elevated refractive index for light trapping.

According to another embodiment, the glass can be float formed as knownin the art of float forming glass.

In one embodiment, the glass is in the form of a sheet. The glass in theform of a sheet can be thermally tempered.

In one embodiment, an Organic Light Emitting Diode device comprises theglass in the form of a sheet.

The glass, according to one embodiment, is transparent. The glass sheet,according to one embodiment, is transparent.

FIG. 1 is an illustration of features 100 of a photovoltaic deviceaccording to one embodiment. In one embodiment, a photovoltaic devicecomprises the glass in the form of a sheet. The photovoltaic device cancomprise more than one of the glass sheets, for example, as a substrateand/or as a superstrate. In one embodiment, the photovoltaic devicecomprises the glass sheet 10 as a substrate and/or superstrate, aconductive material 12 located adjacent to the substrate, and an activephotovoltaic medium 16 adjacent to the conductive material. In oneembodiment, the active photovoltaic medium comprises a copper indiumgallium diselenide (CIGS) layer. In one embodiment, the activephotovoltaic medium comprises a cadmium telluride (CdTe) layer. In oneembodiment, the active photovoltaic medium is a CIGS layer. In oneembodiment, the active photovoltaic medium is a cadmium telluride (CdTe)layer.

The photovoltaic device, according to one embodiment, further comprisesa barrier layer 14 disposed between the superstrate or substrate and theactive photovoltaic medium. In one embodiment, the photovoltaic devicefurther comprises a barrier layer disposed between or adjacent to thesuperstrate or substrate and a transparent conductive oxide (TCO) layer,wherein the TCO layer is disposed between or adjacent to the activephotovoltaic medium and the barrier layer. A TCO may be present in aphotovoltaic device comprising a CdTe functional layer. In oneembodiment, the barrier layer is disposed directly on the glass. Thebarrier layer can effect the migration of alkali ions from the glassinto other layers of the device, for example, the active photovoltaicmedium, for example, increase, decrease, or meter the migration.

In one embodiment, the glass sheet is transparent. In one embodiment,the glass sheet as the substrate and/or superstrate is transparent.

According to some embodiments, the glass sheet has a thickness of 4.0 mmor less, for example, 3.5 mm or less, for example, 3.2 mm or less, forexample, 3.0 mm or less, for example, 2.5 mm or less, for example, 2.0mm or less, for example, 1.9 mm or less, for example, 1.8 mm or less,for example, 1.5 mm or less, for example, 1.1 mm or less, for example,0.5 mm to 2.0 mm, for example, 0.5 mm to 1.1 mm, for example, 0.7 mm to1.1 mm. Although these are exemplary thicknesses, the glass sheet canhave a thickness of any numerical value including decimal places in therange of from 0.1 mm up to and including 4.0 mm.

In one embodiment, an electrochromic device comprises the glass in theform of a sheet. The electrochromic device can be, for example, anelectrochromic window. In one embodiment, the electrochromic windowcomprises one or more of the glass sheets, such as in a single, double,or triple pane window.

The fusion formable glasses of this invention, by virtue of theirrelatively high strain point, represent advantaged substrate materialsfor CIGS photovoltaic modules. When manufactured by the fusion process,their superior surface quality relative to that of float glass may alsoresult in further improvements to the photovoltaic module makingprocess. Advantageous embodiments of this invention are characterized byliquidus viscosity in excess of 400,000 poise, thereby enabling thefabrication of the relatively thick glass sheets that may beadvantageous for some module manufacturers.

EXAMPLES

The following is an example of how to fabricate a sample of an exemplaryglass, according to one embodiment of the invention, as shown inTable 1. This composition corresponds to Example number 1 shown in Table3.

TABLE 1 oxide mol % SiO₂ 63.64 Al₂O₃ 13.00 MgO 3.14 CaO 3.15 SrO 1.56Na₂O 11.32 K₂O 4.09 SnO₂ 0.10In some embodiments, the total does not add up to 100%, since certaintramp elements are present at non-negligible concentrations.

Batch materials, as shown in Table 2 were weighed and added to a 4 literplastic container:

TABLE 2 batch Batch Components weight sand 1322.67 alumina 473.03Magnesia 45.22 Limestone 115.32 Strontium carbonate 83.32 Soda ash425.20 Potassium carbonate 202.74 10% SnO₂ and 90% 52.8 sand

It should be appreciated that in the batch, limestone, depending on thesource can contain tramp elements and/or vary amounts of one or moreoxides, for example, MgO and/or BaO. The sand is advantageouslybeneficiated so that at least 80% by mass passes 60 mesh, for example 80mesh, for example 100 mesh. The SnO₂ added, in this example, waspre-mixed with sand at a level of 10% by weight so as to ensurehomogeneous mixing with the other components. The bottle containing thebatch materials was mounted to a tumbler and the batch materials weremixed so as to make a homogeneous batch and to break up softagglomerates. The mixed batch was transferred to a 1800 cc platinumcrucible and placed into a high-temperature ceramic backer. The platinumin its backer was loaded into a glo-bar furnace idling at a temperatureof 1630° C. After 16 hours, the crucible+backer was removed and theglass melt was poured onto a cold surface, such as a steel plate, toform a patty, and then transferred to an annealer held at a temperatureof 640° C. The glass patty was held at the annealer temperature for 2hours, then cooled at a rate of 1° C. per minute to room temperature.

Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10,Table 11, and Table 12 show exemplary glasses, according to embodimentsof the invention, and made according to the above example. Propertiesdata for some exemplary glasses are also shown in Table 3, Table 4,Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11, andTable 12. In the Tables T_(str)(° C.) is the strain point which is thetemperature when the viscosity is equal to 10^(14.7) P as measured bybeam bending or fiber elongation. T_(ann)(° C.) is the annealing pointwhich is the temperature when the viscosity is equal to 10^(13.18) P asmeasured by beam bending or fiber elongation. T_(s)(° C.) is thesoftening point which is the temperature when the viscosity is equal to10^(7.6) P as measured by beam bending or fiber elongation. α(10⁻⁷/° C.)or a(10⁻⁷/° C.) in the Tables is the coefficient of thermal expansion(CTE) which is the amount of dimensional change from either 0 to 300° C.or 25 to 300° C. depending on the measurement. CTE is typically measuredby dilatometry. ρ(g/cc) is the density which is measured with theArchimedes method (ASTM C693). T₂₀₀(° C.) is the two-hundred Poise (P)temperature. This is the temperature when the viscosity of the melt is200 P as measured by HTV (high temperature viscosity) measurement whichuses concentric cylinder viscometry. T_(liq)(° C.) is the liquidustemperature. This is the temperature where the first crystal is observedin a standard gradient boat liquidus measurement (ASTM C829-81).Generally this test is 72 hours but can be as short as 24 hours toincrease throughput at the expense of accuracy (shorter tests couldunderestimate the liquidus temperature). η_(liq)(° C.) is the liquidusviscosity. This is the viscosity of the melt corresponding to theliquidus temperature.

TABLE 3 Example 1 2 3 4 5 6 Composition (mol %) Na₂O 11.32 10.30 12.3011.32 11.32 11.32 K₂O 4.09 5.11 3.11 3.09 2.09 4.09 MgO 3.14 3.14 3.143.54 3.94 3.94 CaO 3.14 3.14 3.14 3.54 3.94 3.94 SrO 1.57 1.57 1.57 1.771.97 1.97 Al₂O₃ 13.00 13.00 13.00 13.00 13.00 12.00 SiO₂ 63.63 63.6363.63 63.63 63.63 62.63 SnO₂ 0.10 0.10 0.10 0.10 0.10 0.10 Composition(wt %) Na₂O 10.40 9.43 11.40 10.50 10.50 10.50 K₂O 5.74 7.13 4.38 4.362.96 5.78 MgO 1.89 1.88 1.89 2.14 2.39 2.38 CaO 2.63 2.62 2.64 2.98 3.333.32 SrO 2.42 2.41 2.43 2.75 3.07 3.06 Al₂O₃ 19.70 19.60 19.80 19.8019.90 18.30 SiO₂ 56.90 56.60 57.20 57.20 57.50 56.40 SnO₂ 0.22 0.22 0.230.23 0.23 0.23 T_(str) (° C.) 595 591 583 593 603 570 T_(ann) (° C.) 644642 635 646 656 621 α (10⁻⁷/° C.) 87.9 90.2 88.2 83.6 80.5 89.8 ρ(gm/cc) 2.513 2.509 2.512 2.519 2.527 2.534 T₂₀₀ (° C.) 1630 T_(liq) (°C.) 1025 1045 1025 1055 1090 1040 η_(liq) (kp) 546

TABLE 4 Example 7 8 9 10 11 Composition (mol %) Na₂O 11.32 11.32 11.3211.09 10.87 K₂O 4.09 4.09 4.09 4.01 3.93 MgO 3.14 3.14 3.14 3.08 3.01CaO 3.15 3.15 3.15 3.09 3.02 SrO 1.56 1.56 1.56 1.53 1.5 Al₂O₃ 11.009.00 7.00 12.74 12.48 SiO₂ 63.63 63.63 63.63 62.36 61.09 SnO₂ 0.10 0.100.10 0.10 0.10 B₂O₃ 2.00 4.00 6.00 2.00 4.00 (RO + R2O)/Al₂O₃ 2.11 2.583.32 1.79 1.79 R₂O/RO 1.96 1.96 1.96 1.96 1.97 (RO + R₂O)/Al₂O₃ + B₂O₃1.79 1.79 1.79 1.55 1.35 R₂O/Al₂O₃ + B₂O₃ 1.19 1.19 1.19 1.02 0.90Composition (wt %) Na₂O 10.51 10.61 10.72 10.19 9.98 K₂O 5.79 5.85 5.915.62 5.51 MgO 1.90 1.92 1.94 1.85 1.80 CaO 2.66 2.68 2.71 2.58 2.52 SrO2.43 2.45 2.48 2.36 2.31 Al₂O₃ 16.86 13.93 10.94 19.32 18.91 SiO₂ 57.4758.04 58.61 55.73 54.55 SnO₂ 0.23 0.23 0.23 0.22 0.22 B₂O₃ 2.09 4.236.41 2.07 4.14 T_(str) (° C.) 550 539 537 566 550 T_(ann) (° C.) 595 582579 614 595 α (10⁻⁷/° C.) 90.4 87.7 83.6 90.2 87.5 ρ (gm/cc) 2.503 2.5002.494 2.507 2.494 T₂₀₀ (° C.) 1574 1583 T_(liq) (° C.) η_(liq) (kp) 389323

TABLE 5 Example 12 13 14 15 16 17 18 19 Composition (mol %) Na₂O 10.9311.06 11.19 11.19 11.06 10.93 10.93 10.93 K₂O 3.95 3.99 4.04 4.04 3.993.95 3.95 3.95 MgO 0 0 0 3.11 3.07 3.03 0 0 CaO 7.59 7.68 7.77 3.11 3.073.04 7.59 7.59 SrO 0 0 0 1.55 1.54 1.52 0 0 B₂O₃ 0 0 0 0 0 0 1.00 2.00Al₂O₃ 16.00 15.00 14.00 14.00 15.00 16.00 16.00 16.00 SiO₂ 61.43 62.1762.9 62.9 62.17 61.43 60.43 59.43 SnO₂ 0.10 0.10 0.10 0.10 0.10 0.100.10 0.10 Composition (wt %) Na₂O 9.92 10.10 10.30 10.20 10.10 9.88 9.909.89 K₂O 5.46 5.55 5.66 5.63 5.53 5.44 5.46 5.45 MgO 0 0 0 1.86 1.821.79 0 0 CaO 6.25 6.36 6.47 2.58 2.53 2.49 6.24 6.23 SrO 0 0 0 2.38 2.352.30 0 0 B₂O₃ 0 0 0 0 0 0 1.02 2.04 Al₂O₃ 23.90 22.60 21.20 21.10 22.5023.90 23.90 23.90 SiO₂ 54.20 55.20 56.10 55.90 55.00 54.00 53.20 52.30SnO₂ 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 T_(str) (° C.) 630 618 604602 615 628 607 588 T_(ann) (° C.) 684 669 655 653 669 683 659 639 α(10⁻⁷/° C.) 85.9 87.9 89 86.7 85.5 85.2 86.2 86.7 ρ (gm/cc) 2.502 2.5052.504 2.513 2.513 2.515 2.496 2.499 T₂₀₀ (° C.) 1622 1622 1646 1645 16591613 1604 T_(liq) (° C.) 1055 1085 1035 1075 1120 1035 1025 h_(liq) (kP)448 194 729 339 237 539 427

TABLE 6 Example 20 21 22 23 24 25 26 27 Composition (mol %) Na₂O 14.8012.20 14.80 14.80 14.80 14.80 14.00 14.00 K₂O 0.90 2.90 0 0 0.50 0 0 0MgO 3.60 1.30 4.50 3.60 3.60 3.60 0 0 CaO 1.90 5.70 1.90 2.80 1.90 1.908.00 5.00 SrO 0 0 0 0 0.40 0.90 0 0 ZnO 0 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 0 00 0 Al₂O₃ 10.80 14.20 10.80 10.80 10.80 10.80 10.00 11.00 TiO₂ 0 0 0 0 00 0 0 ZrO₂ 0 0 0 0 0 0 0 0 SiO₂ 67.90 63.60 67.90 67.90 67.90 67.9067.90 69.90 SnO₂ 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 Composition (wt%) Na₂O 14.20 11.30 14.30 14.30 14.20 14.20 13.50 13.30 K₂O 1.32 4.09 00 0.73 0 0 0 MgO 2.25 0.78 2.83 2.26 2.25 2.25 0 0 CaO 1.65 4.78 1.672.45 1.65 1.65 6.98 4.33 SrO 0 0 0 0 0.64 1.44 0 0 ZnO 0 0 0 0 0 0 0 0B₂O₃ 0 0 0 0 0 0 0 0 Al₂O₃ 17.10 21.70 17.20 17.20 17.10 17.10 15.9017.30 TiO₂ 0 0 0 0 0 0 0 0 ZrO₂ 0 0 0 0 0 0 0 0 SiO₂ 63.30 57.20 63.7063.60 63.20 63.20 63.50 64.80 SnO₂ 0.23 0.23 0.24 0.23 0.23 0.23 0.230.23 T_(str) (° C.) 582 604 586 581 579 578 573 583 T_(ann) (° C.) 632658 639 632 629 629 622 636 T_(s) α (10⁻⁷/° C.) 84.6 86.6 82.3 83 80.183.8 81 78.9 ρ (gm/cc) 2.451 2.489 2.45 2.452 2.461 2.469 2.489 2.455T₂₀₀ (° C.) 1652 1646 1633 1623 1631 1635 1656 T_(liq) (° C.) 990 10251040 1040 985 1010 1130 1070 h_(liq) (kP) 765 771 360 261 558 552 157

TABLE 7 Example 28 29 30 31 32 33 34 35 Composition (mol %) Na₂O 14.0014.80 14.00 14.00 11.50 12.50 14.80 14.00 K₂O 0 0 0 0 3.9 3.9 0 0 MgO2.00 2.75 0 2.00 0 0 3.60 0 CaO 2.00 1.45 4.00 2.00 7.60 7.60 1.90 0 SrO1.00 0.70 0 1.00 0 0 0.90 5.00 ZnO 0 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 0 0 0 0Al₂O₃ 11.00 12.30 12.00 11.00 16.00 16.00 10.80 11.00 TiO₂ 0 0 0 2.00 00 0 0 ZrO₂ 0 0 0 0 0 0 1.50 0 SiO₂ 69.90 67.90 69.90 67.90 60.90 59.9066.40 69.90 SnO₂ 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 Composition (wt%) Na₂O 13.30 14.00 13.30 13.20 10.40 11.30 14.00 12.90 K₂O 0 0 0 0 5.395.39 0 0 MgO 1.24 1.70 0 1.23 0 0 2.22 0 CaO 1.73 1.25 3.44 1.72 6.266.26 1.63 0 SrO 1.59 1.11 0 1.58 0 0 1.42 7.70 ZnO 0 0 0 0 0 0 0 0 B₂O₃0 0 0 0 0 0 0 0 Al₂O₃ 17.30 19.20 18.70 17.20 24.00 23.90 16.80 16.70TiO₂ 0 0 0 2.44 0 0 0 0 ZrO₂ 0 0 0 0 0 0 2.82 0 SiO₂ 64.60 62.50 64.3062.40 53.70 52.80 60.90 62.40 SnO₂ 0.23 0.23 0.23 0.23 0.22 0.22 0.220.22 T_(str) (° C.) 585 602 599 595 626 624 614 570 T_(ann) (° C.) 639656 654 641 678 669 662 618 T_(s) α (10⁻⁷/° C.) 77.6 79.6 77.8 78.4 88.991.4 79 79.1 ρ (gm/cc) 2.458 2.455 2.44 2.479 2.51 2.516 2.516 2.536T₂₀₀ (° C.) 1681 1682 1725 1605 T_(lig) (° C.) 1030 1040 1040 1035 10801110 1010 1060 h_(liq) (kP) 536 790 848 910

TABLE 8 Example 36 37 38 39 40 41 42 43 Composition (mol %) Na₂O 14.0016.00 14.00 13.00 13.00 13.00 13.00 13.00 K₂O 0 0 0 0 0 0 0 0 MgO 0 5.004.50 4.00 3.50 3.50 3.00 5.90 CaO 0 0 0 4.00 3.50 3.50 3.00 0 SrO 0 0 00 0 0 0 0 ZnO 5.00 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 1.00 0 0 0 Al₂O₃ 11.009.00 8.20 10.00 9.00 10.00 9.00 8.08 TiO₂ 0 0 0 0 0 0 0 0 ZrO₂ 0 0 0 0 00 0 0 SiO₂ 69.90 70.00 73.50 69.00 70.00 70.00 72.00 73.00 SnO₂ 0.100.10 0.10 0.10 0.10 0.10 0.10 0.10 Composition (wt %) Na₂O 13.10 15.6113.70 12.60 12.64 12.58 12.64 12.81 K₂O 0 0 0 0 0 0 0 0 MgO 0 3.18 2.872.53 2.22 2.21 1.9 3.81 CaO 0 0 0 3.52 3.09 3.08 2.65 0 SrO 0 0 0 0 0 00 0 ZnO 6.16 0 0 0 0 0 0 0 B₂O₃ 0 0 0 0 1.11 0 0 0 Al₂O₃ 17.00 14.4913.24 16.00 14.44 15.97 14.44 13.14 TiO₂ 0 0 0 0 0 0 0 0 ZrO₂ 0 0 0 0 00 0 0 SiO₂ 63.50 66.43 69.92 65.06 66.22 65.87 68.08 69.95 SnO₂ 0.230.24 0.24 0.24 0.24 0.24 0.24 0.24 T_(str) (° C.) 617 570 580 600 572594 578 592 T_(ann) (° C.) 671 621 634 650 620 646 631 650 T_(s) 854 886880 851 888 878 915 α (10⁻⁷/° C.) 82.9 76.6 74.3 75.8 74.1 73 71.5 ρ(gm/cc) 2.497 2.429 2.392 2.457 2.443 2.445 2.432 2.404 T₂₀₀ (° C.) 16411731 1641 1637 1664 1678 1687 T_(liq) (° C.) 1010 970 985 1105 1050 10901050 1030 h_(liq) (kP) 1223 2127 104 177 178 338 668

TABLE 9 Example 44 45 46 Composition (mol %) Na₂O 15.00 15.34 11.90 K₂O0 0 0 MgO 0 0 5.00 CaO 3.40 0.59 0 SrO 0 0 0 ZnO 0 0 0 B₂O₃ 0 5.00 2.00Al₂O₃ 11.50 16.03 9.00 TiO₂ 0 0 0 ZrO₂ 0 0 0 SiO₂ 70.00 62.94 72.00 SnO₂0.10 0.10 0.10 Composition (wt %) Na₂O 14.23 14.02 11.6 K₂O 0 0 0 MgO 00 3.18 CaO 2.93 0.49 0 SrO 0 0 0 ZnO 0 0 0 B₂O₃ 0 5.15 2.20 Al₂O₃ 18.0024.17 14.48 TiO₂ 0 0 0 ZrO₂ 0 0 0 SiO₂ 64.57 55.92 68.26 SnO₂ 0.23 0.230.23 T_(str) (° C.) 589 609 591 T_(ann) (° C.) 638 668 645 T_(s) 876 954918 α (10⁻⁷/° C.) 80 80.7 68.1 ρ (gm/cc) 2.445 2.405 2.395 T₂₀₀ (° C.)T_(liq) (° C.) 1010 none 1110 h_(liq) (kP)

TABLE 10 Example 47 48 49 50 51 52 Composition (mol %) Na₂O 14.80 14.8514.88 14.80 14.80 14.80 MgO 3.60 1.38 0.69 3.85 3.60 3.30 CaO 1.90 3.734.86 2.05 1.90 1.75 SrO 0.90 0.35 0.18 1.00 0.90 0.85 Al₂O₃ 10.80 13.1513.58 12.30 12.80 13.30 ZrO₂ 1.00 0 0 0 0 0 SiO₂ 66.90 66.44 65.71 65.9065.90 65.90 SnO₂ 0.10 0.10 0 0.10 0.10 0.10 Composition (wt %) Na₂O14.00 14.00 14.00 14.00 14.00 13.90 MgO 2.23 0.85 0.42 2.38 2.22 2.03CaO 1.63 3.19 4.14 1.76 1.63 1.49 SrO 1.43 0.55 0.28 1.59 1.43 1.34Al₂O₃ 16.90 20.40 21.00 19.20 20.00 20.70 ZrO₂ 1.89 0 0 0 0 0 SiO₂ 61.6060.80 59.90 60.70 60.50 60.30 SnO₂ 0.23 0.23 0.23 0.23 0.23 0.23 T_(str)(° C.) 607 612 616 606 621 623 T_(ann) (° C.) 654 663 666 657 670 675 Tsα (10⁻⁷/° C.) 79.8 80.5 81.3 79.4 78.8 79.6 ρ (gm/cc) 2.501 2.474 2.4792.482 2.476 2.474 T₂₀₀ (° C.) 1628 1641 1668 T_(liq) (° C.) 1015 10501045 1060 1070 1080 η_(liq) (kp) 817 358 475

TABLE 11 Example 53 54 55 56 57 58 Com- position (mol %) Na₂O 13.5014.90 13.19 14.90 15.58 16.93 K₂O 0 0 0.98 0 0 0 MgO 0 0 5.68 0 0 0 CaO4.50 6.00 1.17 2.00 4.92 1.60 SrO 0 0 0.38 0 0 0 ZnO 0 0 0 4.00 0 0Al₂O₃ 12.00 14.00 9.50 14.00 15.34 18.57 ZrO₂ 0 0 0 0 0 0 SiO₂ 70.0065.00 67.31 65.00 64.09 58.80 SnO₂ 0.10 0 0.10 0.10 0.10 0.01 Com-position (wt %) Na₂O 12.77 13.93 12.87 13.73 14.42 15.25 K₂O 0 0 1.46 00 0 MgO 0 0 3.62 0 0 0 CaO 3.86 5.09 1.04 1.67 4.14 1.31 SrO 0 0 0.62 00 0 ZnO 0 0 0 4.85 0 0 Al₂O₃ 18.72 21.60 15.29 21.28 23.44 27.6 ZrO₂ 0 00 0 0 SiO₂ 64.38 59.10 63.87 58.23 57.67 51.5 SnO₂ 0.23 0.23 0.23 0.230.23 0.23 T_(str) (° C.) 613 619 588 623 636 617 T_(ann) (° C.) 666 669637 675 686 675 Ts 911 899 872 924 919 937 α (10⁻⁷/ 75.6 81.6 80.1 79.482.4 83.1 ° C.) ρ (gm/cc) 2.449 2.484 2.549 2.519 2.482 2.442 T₂₀₀ (°C.) 1744 1641 1679 1634 T_(liq) (° C.) 1040 1050 1000 1035 1040 1050η_(liq) (kp) 985 521 1927 1279

TABLE 12 Example 59 60 61 62 63 64 Composition (mol %) Na₂O 12.57 12.2710.77 11.07 10.82 11.58 K₂O 2.30 2.24 2.24 3.44 2.79 2.30 MgO 4.81 4.694.69 3.39 3.64 6.20 CaO 2.60 2.00 3.00 3.15 3.15 0.60 SrO 0 1.00 1.501.56 1.56 0 ZnO 0 0 0 0 0 0 Al₂O₃ 8.70 10.44 10.44 12.15 11.30 7.61 ZrO₂0 0 0 0 0 0 SiO₂ 68.82 67.16 67.16 65.13 66.63 71.51 SnO₂ 0.20 0.20 0.200.10 0.10 0.20 Composition (wt %) Na₂O 12.17 11.65 10.20 10.28 10.1411.32 K₂O 3.39 3.24 3.24 4.87 3.99 3.43 MgO 3.04 2.91 2.90 2.05 2.233.96 CaO 2.28 1.72 2.58 2.66 2.68 0.53 SrO 0 1.59 2.38 2.43 2.45 0 ZnO 00 0 0 0 0 Al₂O₃ 13.88 16.36 16.32 18.62 17.48 12.27 ZrO₂ 0 0 0 0 0 0SiO₂ 64.73 62.01 61.86 58.80 60.74 67.97 SnO₂ 0.47 0.46 0.46 0.23 0.230.48 T_(str) (° C.) 560 584 598 592 593 571 T_(ann) (° C.) 610 634 648642 644 622 Ts 837.6 866.1 876.6 871.8 876.8 866.8 α (10⁻⁷/° C.) 86.585.2 81.2 88.3 84.9 82.4 ρ (gm/cc) 2.459 2.483 2.498 2.507 2.500 2.428T₂₀₀ (° C.) 1629 1641 1650 1630 1663 1677 T_(liq) (° C.) 1005 1040 10901040 1070 915 η_(liq) (kp) 370 384 143 338 282 4800

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A photovoltaic device, comprising a glassconsisting essentially of in weight percent: 50 to 72 percent SiO₂;greater than 15 to 25 percent Al₂O₃; 0 to 10 percent B₂O₃; 10 to 25percent total M₂O; greater than 0 to 25 percent total RO; wherein M isan alkali metal selected from Na, K, Li, Rb, and Cs, wherein the glasscomprises at least 9 weight percent Na₂O, and wherein R is an alkalineearth metal selected from Mg, Ca, Ba, and Sr, wherein the glasscomprises less than 4 weight percent K₂O, the glass is in the form of asheet and is a substrate or superstate; and an active photovoltaicmedium adjacent to the substrate or supersrate, wherein the glass has astrain point of 535° C. or greater, a coefficient of thermal expansionof 50 ×10⁻⁷ or greater, and a liquidus viscosity of 130,000 poise orgreater.
 2. The photovoltaic device according to claim 1, wherein theactive photovoltaic medium comprises copper indium gallium diselenide orcadmium telluride.
 3. The photovoltaic device according to claim 1,wherein the glass comprises 0.5 to less than 14 weight percent RO. 4.The photovoltaic device according to claim 3, wherein the glasscomprises less than 2.5 weight percent MgO.
 5. The photovoltaic deviceaccording to claim 1, wherein the glass comprises 55 to 72 weightpercent SiO₂.
 6. The photovoltaic device according to claim 1, whereinthe glass comprises 9 to 17 weight percent Na₂O.
 7. The photovoltaicdevice according to claim 1, comprising a barrier layer between theactive photovoltaic medium and the substrate or superstrate.
 8. Thephotovoltaic device according to claim 1, wherein the glass comprisesless than 2.5 weight percent MgO.
 9. The photovoltaic device accordingto claim 1, wherein the glass comprises 0 to 3 percent K₂O.
 10. Thephotovoltaic device according to claim 1, wherein the glass issubstantially free of K₂O.
 11. The photovoltaic device according toclaim 1, wherein the glass comprises no B₂O₃.
 12. The photovoltaicdevice according to claim 1, wherein the glass comprises 21 to 25percent Al₂O₃.
 13. The photovoltaic device according to claim 12,wherein the glass comprises 0 to 3 percent K₂O.
 14. The photovoltaicdevice according to claim 12, wherein the glass is substantially K₂Ofree.
 15. The photovoltaic device according to claim 12, wherein theglass comprises no B₂O₃.
 16. A glass consisting essentially of in weightpercent: 50 to 72 percent SiO₂; 21 to 25 percent Al₂O₃; 0 to 10 percentB₂O₃; 10 to 25 percent total M₂O; and greater than 0 to 25 percent totalRO; wherein M is an alkali metal selected from Na, K, Li, Rb, and Cs,wherein the glass comprises at least 9 weight percent Na₂O, wherein R isan alkaline earth metal selected from Mg, Ca, Ba, and Sr, and whereinthe glass comprises less than 4 weight percent K₂O, wherein the glasshas a strain point of 535° C. or greater, a coefficient of thermalexpansion of 50 ×10⁻⁷ or greater, and a liquidus viscosity of 130,000poise or greater.
 17. The glass according to claim 16, comprising 0 to 3percent K₂O.
 18. The glass according to claim 16, substantially free ofK₂O.
 19. The glass according to claim 16, comprising no B₂O₃.